Elemental Analysis of Organic Samples

ABSTRACT

A method of imaging analyte elements in an organic sample includes providing the sample as a layer on a substrate and reacting the sample on the substrate to produce one or more volatile products that leave the sample while the one or more elements remain in the sample. A majority of the sample layer by weight is removed from the substrate by the reaction and the remaining sample layer is enriched in the one or more elements which are not spatially disturbed by the reaction. The method including subsequently detecting the one or more elements in the concentrated sample layer using an imaging elemental analyzer.

FIELD

The invention relates to the field of imaging mass spectrometry, inparticular imaging elemental mass spectrometry. In certain aspects, theinvention relates to analysing the distribution of particular elementsin a biological sample, which may have been introduced into the sampleas elemental tags or may occur in the sample naturally

BACKGROUND

The distribution of so-called inorganic elements in biological samplesis important to determine for numerous reasons. Inorganic elementsgenerally refers to elements other than those that typically formorganic material such as C, H, N and O. Usually, the inorganic elementsof interest are heavier than oxygen and typically are metallic orsemi-metallic elements. The natural distribution of inorganic elementsin biological samples reveals important information about biologicalprocesses at gene, protein and metabolite levels as reflected by theburgeoning field of metallomics. In addition, in an approach calledelemental tagging, a number of so-called elemental tags (which may alsobe termed markers) can be added artificially to targets in the sample,typically with the help of specific binding agents (for exampleantibodies, aptamers, metabolic labels, etc.) to focus on specifictargets or processes in biological systems. Many different detectiontechniques can be employed for measuring the abundance of the elementsof such tags, such as radioactivity, light (e.g. fluorescence orabsorption), which includes X-ray fluorescence (XRF), secondary electronspectrometry (SES), X-ray photoelectron spectroscopy (XPS), electronmicro-probe analysis (EMPA), secondary ion mass spectrometry (SIMS),laser plasma ionisation mass spectrometry (LPI MS) andinductively-coupled plasma mass spectrometry (ICP MS), etc.

In the case of fluorescence based assays, the techniques may be fast butsuffer from low sensitivity and are limited to one or a few targets perassay in comparison to mass spectrometric techniques such as SIMS or ICPMS.

Mass spectrometry techniques allow a high degree of multiplexedmeasurement of elements in parallel, for example using multi-collectormagnetic sector, time-of-flight, ORBITRAP or Fourier transform ioncyclotron resonance analyzers. However, when spatially resolved analysisis required, for example for imaging of tissues, low abundance ofelements poses a challenge to all these analyzers as spectra becomedominated by intense matrix peaks from tissues. These matrix peaks couldoriginate from polyatomic species constituting bulk of tissues, withmajor elements being not only C, H, N, O, but also S, P, alkali metals(Na, K), etc. Although polyatomic species could in principle beeliminated in RF-only gas-filled reaction cells (e.g. U.S. Pat. No.5,767,512, U.S. Pat. No. 7,230,232), such reactions are highly analytedependent, could affect metals of interest and generally result inlosses of these ions of interest. This is especially noticeable forimaging applications where the starting amount of analyte is limitedfrom the start.

ICP MS with laser ablation (LA/ICPMS) is known to have a negligiblecontribution of polyatomic species and therefore became one of preferredmethods for elemental imaging of tissues as shown for example inWO2010/133196, DE10354787, WO0151907, WO02054057, U.S. Pat. No.8,274,735 WO 2014/063246, WO2015128490 and others. An acquisition rateof up to several tens of pixels/second with micrometer (μm) spatialresolution has been demonstrated. Even with such rate, several hours arestill needed for the acquisition of a single image. Further increases inacquisition speed, however, are limited by the temporal spreading of thesignal due to spreading of the sample plume during its transport fromthe surface to the ICP torch, as most of the transport process takesplace at atmospheric pressure and at low transport velocities.Atmospheric pressure is essential for ICP operation. Along with thisspreading, transfer lines may get coated with aerosol formed by samplematerial, thus resulting in carryover and contamination of the sampleintroduction unit. With higher throughput required by any clinicalapplication, excessive contamination will drive the costs of analysisand service time.

Transfer of the ionisation process into vacuum as known in the art forSIMS or laser plasma ionisation approaches results in a very longscanning process due to a relatively low current of generated ions ofinterest and hence long exposure times being required.

Such low current of generated ions is often caused not so much byionising agent or low efficiency of secondary ion generation but ratherby the relatively low concentration of natural elements or tags in thecell/tissue matrix. This also precludes utilising other methods ofmulti-channel elemental imaging such as SES, micro X-ray fluorescence(μXRF), etc. Another problem is the rapid contamination of the vacuumchamber and analyzer components with the organic matrix material. Forexample, analysis of just one typical 5 μm-thick tissue section of 100mm² area could completely contaminate an instrument if fully utilisedfor analysis in order to satisfy sensitivity requirements. In the caseof SIMS, there is an added problem of relatively slow rate of sampleremoval that decelerates analysis of typical tissue samples which areoften at least 3-5 micrometers thick.

In the field of isotope ratio mass spectrometry (IRMS), especially wherethe isotope ratio analyzer is interfaced to a gas chromatography (GC) orliquid chromatography (LC) separation stage, samples are oxidised toproduce gases such as CO₂, NO_(x), H₂O, which are analysed to determineisotope ratios of elements such as C, N and/or O. The oxidation may takeplace in a combustion oven (e.g. in GC-IRMS), as described in Z. Muccioand G. P. Jackson, Isotope ratio mass spectrometry, Analyst 134 (2009)213-222, or it may involve a wet chemical oxidation process (e.g. inLC-IRMS), as described in C. Osburn and G. St-Jean, Limnology andOceanography: Methods 5 (2007) 296-308. “Dry” oxidation e.g. byUV-ozone, is also routinely used for removal of contaminations onsurfaces of semiconductors, glass, etc.

The present invention has been made against this background.

SUMMARY

The invention relates to an approach that can be used for processingtissue samples prior to their analysis by any of the above-mentionedmethods. As the thickness of tissues is typically quite comparable tothe required spatial resolution of analysis, carefully controlledconditions of oxidation can result in gradual removal of organic matrixwhile limiting diffusion of heavier elements or tags away from theiroriginal position. As a result, a much lower amount of material will bedesorbed or removed during sampling, at the same time deliveringrequired analytes of interest and imaging information.

The present invention provides an improved approach to elementalanalysis of organic samples by a pre-concentration of elements to beanalysed (which may be variously termed herein elements of interest oranalyte elements). These are typically the inorganic elements present inthe sample, either naturally or through introduction as a tag. Thisenables a variety of methods for elemental imaging to be used that havebeen ineffective in earlier approaches.

According to one aspect of the invention there is provided a method ofimaging one or more analyte elements in an organic sample, comprising:

providing the sample as a layer on a substrate;

reacting (preferably oxidizing) the sample on the substrate to produceone or more volatile products that leave the sample and enter the gasphase, whilst the one or more analyte elements remain in the sample,whereby a majority of the sample layer by weight is removed from thesubstrate by the reaction (preferably oxidation) and the remainingsample layer is enriched or concentrated in the one or more analyteelements; and

detecting the one or more analyte elements in the enriched orconcentrated sample layer using an imaging elemental analyzer.

Preferably, the analyte elements are not spatially disturbed by thereaction by more than the spatial resolution of the imaging analysis.Although some individual analytes may be disturbed by a greater distancethan this, preferably on average the analyte elements are spatiallydisturbed by the reaction not more than the spatial resolution of theimaging analysis.

According to another aspect of the invention there is provided anapparatus for imaging one or more analyte elements in an organic sample,comprising:

a reaction chamber (preferably an oxidation chamber) to receive thesample, wherein the sample has been provided as a layer on a substrate;

wherein the reaction (preferably oxidation) chamber comprises anelectromagnetic radiation source and/or an inlet for introducing intothe chamber one or more chemical or ionic oxidizing agents for oxidizingthe sample to produce one or more volatile products that leave thesample and enter the gas phase, whilst the one or more analyte elementsremain in the sample, whereby a majority of the sample layer by weightis removed from the substrate by the oxidation and the remaining samplelayer is enriched in the one or more analyte elements; and

an imaging elemental analyzer in a detection chamber for detecting thespatial distribution of said one or more analyte elements in theenriched sample layer.

According to a further aspect of the invention there is provided adedicated imaging elemental analyzer for imaging one or more analyteelements in an organic sample, the analyzer comprising:

a chamber for housing an organic sample containing one or more analyteelements to be imaged, wherein the pressure inside the chambersurrounding the sample is in the range 10⁻⁵ to 10⁻² mbar;

at least one primary irradiation means selected from: (i) an ion gun forirradiating the sample with a high intensity beam of primary ions,wherein the primary ions are formed in the ion gun at a pressure below 1mbar, wherein the ion gun is for focusing the beam of primary ions to alocalized spot on the surface of the sample and for moving the spot to aplurality of locations on the surface of the sample over time; (ii) alaser, preferably high-power laser, for irradiating a localized spot onthe surface of the sample to produce ions and for moving the spot to aplurality of locations on the surface of the sample over time;

a gas-filled RF ion guide for receiving produced ions comprising theanalyte elements released from the sample in response to the primaryirradiation, wherein the RF ion guide prevents onward transmission ofall ions of m/z below the mass or mass range of the analyte elements;preferably at least some of produced ions undergoing ion-moleculereaction in said ion guide; and

a time of flight (TOF) mass analyzer for receiving the produced ions orreaction products of the produced ions from the RF ion guide, whereinthe TOF mass analyzer is configured to have a repetition rate of atleast 5 kHz, preferably 50-100 kHz.

According to an additional aspect of the invention, there is provided anelemental analyzer for mass analyzing, and preferably imaging, one ormore analyte elements in a sample, the analyzer comprising:

-   -   a chamber for housing a sample containing one or more analyte        elements, preferably wherein the pressure inside the chamber        surrounding the sample is in the range 10⁻⁵ to 10⁻² mbar;    -   a laser for irradiating a localized spot on the surface of the        sample and causing laser plasma ionisation of at least one or        more analyte elements in the sample, preferably wherein the        laser is for moving the spot to a plurality of locations on the        surface of the sample over time;    -   a reaction cell for receiving ions of the one or more analyte        elements produced by laser plasma ionisation, wherein a        composition and emittance of the ions is altered, preferably        reduced, as the ions travel through the reaction cell; and    -   a mass analyzer, preferably a time of flight (TOF) mass        analyzer, for receiving ions of the one or more analyte elements        and/or ions of reaction products of the one or more analyte        elements from the reaction cell, preferably wherein the TOF mass        analyzer is configured to have a repetition rate of at least 5        kHz.

Preferred Embodiments

A pre-concentration of analyte elements, which may be tags, on thesubstrate is implemented by enabling oxidation reactions that convertthe organic matrix or material of the sample into volatile gases thatare removed to waste, while inorganic elements of interest remain on thesubstrate. Accordingly, preferably the volatile products substantiallydo not contain the analyte elements. Furthermore, the inorganicelemental species may end up in oxidised form on the substrate (i.e. inthe oxidised sample).

The enriched analyte elements in the remaining sample may then bedetected using an imaging elemental analyzer. The detection can takeplace at a different time and location (e.g. in a different chamber) tothe oxidation. For example, the detection typically takes placesubsequently to the oxidation. From the detection, an image may then begenerated of the detected elements in the sample. The imaging elementalanalyzer may thus comprise a data acquisition system that receives inputfrom the detection of the one or more elements and generates an image ofthe one or more elements in the sample. The imaging elemental analyzeris desirably a device capable of rapid imaging of multiple elements inparallel, such as a device comprising a mass analyzer or polychromator.

The initial sample is an organic sample, i.e. comprised of mostlyorganic matter and containing a minor or trace amount of inorganicmatter including the analyte elements to be detected. It may be anysample comprising an organic matrix in which one or more analyteelements are contained, which are desired to be detected. The organicmatrix constitutes the majority of the mass or weight of the sample. Theorganic matrix may constitute at least 60%, or at least 70%, or at least80%, or at least 90%, or at least 95%, or at least 99%, or at least99.9%, or at least 99.99%, of the sample layer by weight.

The sample may be a biological sample, i.e. of biological origin. Thebiological sample may be derived from an organism. The organism may beplant or animal or bacteria. In a preferred application of theinvention, the biological sample is tissue and/or individual cells.

The analyte elements are generally elements other than those thattypically form the organic matrix (C, H, N and O). Typically, theanalyte elements are heavier than oxygen. Typically, the elements aremetallic or semi-metallic (metalloid) elements. The elements preferablymay be metals heavier than mass 16. The elements may be heavy metalelements. The elements may be selected from rare-earth elements(lanthanides) or transition metals or post-transition metals, or alkalimetals, or alkaline earth metals or metalloids. The elements may beradioisotopes. In the case of a plurality of analytes, the elements maybe any combination of the above classes. The one or more analyteelements may comprise two or more different isotopes of the sameelement.

The one or more analyte elements may be naturally occurring in thesample, e.g. as trace elements in a sample, such as a biological sample.Such elements could be also used as internal standards for improvedquantitation. The one or more analyte elements may have been introducedinto the sample as elemental tags, e.g. using methods of elementaltagging known in the art. One class of preferred elemental tags israre-earth elements (particularly lanthanides). The tags may beradioisotopes, detectable by a radioactivity analyzer.

The one or more elemental tags may be provided as nanoparticles,nanorods, mass dots or quantum dots, for example as described in US2014/0221241 A1.

The one or more elemental tags may be provided as purified isotopes ofrare-earth or other elements or combinations of them in a pre-determinedratio.

The one or more elemental tags may be attached to a binding member thatbinds to a target in the sample. The binding member may be specific sothat it binds to a specific target in the sample. Each elemental tag(each mass), where there is more than one, may be attached to adifferent binding member that is specific for a particular target in thesample. Thus, a plurality of different targets may be present.Preferably each elemental tag is bound to a different specific bindingmember. The one or more elemental tags may be attached to the bindingmember directly or indirectly (e.g. via a linker). The binding membermay be selected from a stain (e.g. fluorescent stain), polypeptide,polynucleotide, antibody, affibody, and an aptamer), or a SOMAmer™. Thetarget may be any organic molecule in the sample. In the case ofbiological samples, the target may be a biomolecule, for example, amacromolecule such as selected from proteins, polysaccharides, lipids,and nucleic acids, as well as small molecules such as metabolites andnatural products. The target may be an antigen. As an example, theelemental tag may be attached to an antibody so that it becomes attachedto an antibody-antigen complex after the antibody binds to an antigen.The, or each, target is preferably a biomarker.

In some embodiments, the one or more elemental tags may have beenmetabolically introduced into the sample, e.g. within food or supportmedia. The elemental tag may therefore form part of a metabolic label.

The tagging can also utilise multiple elements in a barcode manner, forexample as described in US 2014/106976 and in B. Bodenmiller et al.,Nature Biotechnology 30 (2012) 858-867.

The one or more elemental tags may comprise two or more differentisotopic tags of the same element.

The sample is prepared as a layer on a substrate. The sample ispreferably provided as a thin layer, more preferably no more than (i) 20μm, or (ii) 10 μm, or (iii) 5 μm, or (iv) 3 μm in thickness.

The substrate is typically a slide, e.g. a planar slide. The substrateor slide may be a metal, glass or ceramic flat plate. In someembodiments, the substrate may have a surface of titanium dioxide. Forexample, any of the aforesaid slides or flat plates may have a surfaceof titanium dioxide. For this purpose, the substrate may be coated witha layer of titanium dioxide, preferably in the form of a titaniumdioxide film or immobilised titanium dioxide particles. One of preferredembodiments of substrate is a standard microscope glass slide withindium-tin oxide coating as known in the art.

In some embodiments, the sample may comprise a fixed and embedded tissuesample, e.g. a formalin-fixed, paraffin-embedded (FFPE) tissue,preferably cut by a microtome, preferably to 3-5 μm thickness.

In some embodiments, the sample may comprise individual cells depositedon a substrate, e.g. from a flow cytometer or high-content screeningdevice. The cells may be deposited e.g. in a grid-like pattern (forinstance, at every 50 μm, or every 30 μm in X and Y directions). Atypical cell size in this case may be up to 5 μm or up to 10 μm, or 5-10μm. An example of grid-like sample preparation is shown inWO2014/063246.

In some embodiments, the sample may comprise cell culture on a growthmedia, e.g. a microbial or bacterial culture on a thin layer of growthmedia such as agarose. Preferably, the culture is up to 10 μm, or up to20 μm thick. Typically, growth media will be thicker than this culturethickness. The culture samples could be oxidized as they are (e.g. usingplasma etching), but oxidation and subsequently sampling will not be soeffective for thick layers of growth media. Preferably, such a sampleshould be cut down to the thin culture layer or close to the thinculture layer, to improve oxidation and sampling.

In some embodiments, the sample may be deposited by an autosampler (e.g.including autosamplers of the following types: flow focusing, acousticdroplet ejection, induction, etc.).

The sample may be deposited on the substrate, e.g. by the autosampler,as individual droplets in or on a grid-like pattern (e.g. every 30-50 μmin X and Y direction), or in or on microarrays or in a multi-well plate.

In some embodiments, a plurality of samples (which may be differentsamples) could be deposited on one substrate in different locations onthe substrate, e.g. in a grid-like pattern, as mentioned above.

The tagging of the sample with one or more analyte elements may takeplace before or after providing the sample on the substrate, preferablyafter.

In certain embodiments of the invention, the analyte elements are notelemental tags but naturally occurring elements in the sample (so-callednative elements). Thus, in certain embodiments, the sample is leftunprocessed in the sense of not being tagged. This type of method may beused in applications to yield the distribution of native inorganicelements, such as metal elements, in the sample, particularly nativeheavier inorganic elements (e.g. metals, e.g. Fe, Zn, Sn, etc., e.g. formetallomics experiments).

Once preparation of the sample on the substrate is finished, the samplecan be transferred to a processing chamber (e.g. the oxidation chamber)for the oxidation step. Sample could be optionally lyophilised prior tothis transfer to reduce amount of water in it. The sample may betransferred to a hermetic reaction chamber where it is subjected to oneor more, preferably strong, oxidising agents. In one embodiment,oxidation could comprise heating the sample in a flow or atmosphere ofoxygen to effect combustion. Many different combustion or oxidationprocesses may work as long as the analyte elements or tags are notspatially disturbed by the process more than the desired spatialresolution of the analysis. The finer the spatial resolution of theanalysis (e.g. 1 micron or 3-5 micron spatial resolution may betypically used), the gentler the oxidation process should be. Under nocircumstances is the formation of gas bubbles or boiling allowed as itwould drastically disturb the original spatial distribution of elements.In some embodiments, when the required spatial resolution is of theorder of tens of microns, more violent and rapid oxidations may be used.Though gas-phase oxidation is preferred, wet chemical oxidation andetching by RF discharge plasma could be also implemented as long as therequirement of low disturbance of spatial distribution of elemental tagsremains fulfilled. A combination of several oxidation processes could beused to accelerate enrichment of the remaining sample.

The hermetically sealed chamber may be a reaction chamber that isseparate from a detection chamber wherein the detecting or analysistakes place, or it may be the same chamber as the detection or analysischamber. Accordingly, in some embodiments, the oxidation chamber is thesame chamber as the detection chamber, i.e. there is a single oxidationand detection chamber in such cases. Preferably, the oxidation isperformed in a different chamber to the detection chamber in which theimaging elemental analyzer is located. Accordingly, the oxidized sampletypically has to be transferred from the reaction (i.e. oxidation)chamber to the detection/analysis chamber (in which the imagingelemental analyzer is located). Thus, in such cases, once the oxidationprocess is finished, the substrate (slide) is transferred into a vacuumchamber for elemental imaging by any one of numerous applicable methods(i.e. the process overall is a two step process:conversion/concentration followed by vacuum-based imaging analysis).

Preferably, the oxidizing step comprises exposing the sample to one ormore oxidising agents, optionally wherein the sample is heated duringthe oxidizing step. The apparatus, e.g. the reaction chamber, maytherefore further comprise a heater for heating the sample during theoxidizing step. The reaction chamber may be a vacuum chamber. Theoxidation may be performed in the chamber at elevated (aboveatmospheric) pressure, at atmospheric pressure, or at reduced pressure,i.e. less than atmospheric pressure. The reduced pressure regime may bebetween 100 and 1000 mbar, or between 1 and 100 mbar. In the lattercase, a DC or RF gas discharge may be used to facilitate oxidation. Theone or more oxidizing agents may be selected from (i) electromagneticradiation and/or (ii) one or more gas-phase chemical oxidizing agentsand/or (iii) ions or electrons and/or (iv) one or more liquid-phasechemical oxidizing agents. Thus, connected to the inlet may be a sourceof one or more chemical oxidizing agents, preferably wherein the one ormore oxidizing agents are selected from: ozone, hydrogen peroxide, and,as an example of liquid-phase oxidizing agents, a persulfate (e.g.ammonium, sodium or potassium persulfate). The latter is preferably usedwith a catalyst, e.g. phosphoric acid and silver nitrate. The agents maybe admitted to the chamber at a reduced pressure (less than atmospheric)together or sequentially.

Preferred gas-phase oxidising agents include ozone and hydrogenperoxide. The sample may be oxidised by the action of electromagneticradiation, in particular light, especially light with wavelength <400 nm(preferably UV light but in some embodiments X-rays). For ozoneoxidation example, a UV-ozone oxidation chamber could be used atatmospheric or elevated pressure as known in the art, with additionalfeeding of wet air or wet oxygen and supplementary activation with 254nm and/or 185 nm UV light. When oxidation is carried out using light,the sample optionally resides on a photocatalytic surface of thesubstrate (preferably titanium dioxide surface), which is subjected tothe light. The oxidising light preferably irradiates the surface atintensities above 0.1, or above 1.0, or above 10 milliWatt/cm², or inthe range 0.1-10 milliWatt/cm².

Accordingly, the one or more oxidizing agents can be: (i)electromagnetic radiation and the oxidizing step comprises irradiatingthe sample with light of wavelength less than 400 nm wherein thesubstrate acts as a photocatalyst to oxidize the sample, or (ii) one ormore chemical oxidizing agents and the oxidizing step comprises exposingthe sample to one or more chemical oxidising agents selected from: ozoneand hydrogen peroxide.

A similar effect is achieved by contacting the sample with low-pressureRF or DC gas discharge so that the surface of the sample is bombarded bycharged particles from plasma and oxidation is supplemented bysputtering. This process is typically faster and more “abrasive”comparing to UV-ozone treatment and therefore more suitable for largerlumps or crystals of elements.

The oxidation processes described are suitable to cause rapid oxidationof the organic matrix atoms: e.g. any one or more of the followingoxidation reactions: C→CO, CO₂, N→NO, NO₂, H→H₂O, etc. Thus, the one ormore volatile products preferably comprise one or more oxides of C, H,and/or N. Optionally S is converted to SO₂ or other oxides of sulphur.The generated volatile products of the oxidation are preferably pumpedaway, thus carrying away a portion, preferably most, of the sample mass.Simultaneously, the analyte elements, e.g. heavier elements (heavierthan O), and especially the metallic elements, do not form volatileproducts and therefore remain on the continuously-thinning layer ofsample, mainly in an oxidised form. Thus, the sample becomes or enrichedin the one or more analyte elements prior to analysis by the imagingelemental analyzer. Preferably, the location of the analyte elements inthe sample on the substrate does not change (at least not significantlyor substantially) as a result of the reaction. For this purpose, thereaction rate may be controlled so that no bubbling or boiling takesplace and the location of the analyte elements is not changed. Diffusionis low for heavier elements or larger lumps or crystals of suchelements, but the reaction rate should be chosen low enough to keep thelength of diffusion below a) lx sample thickness D, b) 0.5*D, c) 2*D.Thereby, the image of the analyte elements in the sample so determinedrepresents the distribution of the analytes elements in the originalsample (prior to reaction).

Preferably, the reaction step removes the majority of the sample layerby weight from the substrate. More preferably, in order of preference,the reaction removes at least 60%, or at least 70%, or at least 80%, orat least 90%, or at least 95%, or at least 99%, or at least 99.9%, or atleast 99.99% of the sample layer by weight. In some embodiments, thereaction step removes between 90% and 99% or between 90% and 99.9% ofthe sample layer by weight.

Preferably, the reaction step is continued until the oxidation processsubstantially reaches saturation, or close to it, such that most of theorganic matrix is removed (most preferably >90% or >95% or >99% byweight), and the typically heavier analyte elements are sufficientlyconcentrated for the subsequent analysis.

Preferably, the reaction step comprises controlling the rate of theoxidation process by regulating the supply of oxidising agents(including light or ions where used) and/or temperature of the sample.In some embodiments, the production of at least one of the volatileproducts and/or their concentration in the gas phase can be monitoredfor process control (e.g. using one or more gas sensors), e.g. todetermine the time to stop the oxidation reaction (to monitorcompleteness of oxidation so that the share of undesired products isminimised), or speed of oxidation (so that it is not too violent, whichmay disturb the position of the elements or tags), as well as fordiagnostics, e.g. to measure the relative content of certain elements inthe volatile products or their isotopes to obtain additional types ofinformation about the sample, such as side products, contaminants etc.Another embodiment of reaction chamber includes using sample slides madefrom porous inorganic material through which oxidizing agents (ozone,hydrogen peroxide, persulfate) are forced from a supply underneath. Thisapproach relies on fast diffusion of these agents through the thintissue section and therefore is more likely to result in bubbling andboiling. However, in this case another porous slide can be located justa few micrometers away from the tissue to “catch” non-volatile oxides ofheavier elements carried off by resulting gas flow. This pass-throughapproach allows faster oxidation process without any loss of analytes ofinterest even when bubbles are formed.

Accordingly, in some embodiments, the step of reacting the sample on thesubstrate comprises passing one or more oxidizing agents from anopposite side of the substrate to the sample through pores in thesubstrate to reach the sample, there being a second substrate inproximity to but spaced apart from and facing the sample, whereby theone or more oxidizing agents diffuse through the sample producingvolatile products from the sample and causing non-volatile analyte(heavier) elements and/or their oxides to reach and remain on thesurface of the second substrate for detection by the imaging analyzer.With a sufficiently small gap between the two substrates, e.g. 5-10micrometers, the gap is such that the spatial distribution of theanalyte heavier elements in the sample is substantially preserved afterthe elements are transferred to the second substrate. At leastpreferably on average the analyte elements transferred to the secondsubstrate are spatially disturbed by the oxidation not more than thespatial resolution of the imaging analysis to be performed.

The imaging elemental analyzer may be selected from: a secondaryelectron spectrometer (SES), an X-ray photoelectron spectrometer (XPS),an X-ray fluorescence (XRF) spectrometer, energy dispersive X-raymicroanalyzer, a radioactivity analyzer, ion mobility analyzer, and amass spectrometer (MS), preferably a mass spectrometer.

Preferably, detecting the one or more elements comprises irradiating theconcentrated sample with a beam of primary particles, such as ions orphotons, focused to a localized spot on the surface of the sample toemit secondary particles from the spot and analysing the secondaryparticles to determine the presence and optionally quantity of the oneor more elements at the spot, wherein the spot is moved to a pluralityof locations on the surface of the sample over time, thereby to obtainan image of the one or more elements in the sample wherein each locationof the spot on the surface of the sample corresponds to a pixel of theimage. At least some of the secondary ions are ions that comprise theone or more of the analyte elements. The secondary particles may beanalysed directly as they are emitted from the sample surface (e.g. ifthe secondary particles are elemental ions already, or their oxide ions,and the analyzer is a mass spectrometer; or if the secondary particlesare photons or electrons that are emitted from the one or more analyteelements being characteristic of the or more analyte elements) or theymay be converted into another form for analysis, e.g. converted fromemitted neutral or ionic polyatomic particles into monoatomic elementalions for mass analysis (e.g. in an ICP ion source of a mass analyzer),or converted into reaction products (via ion-molecule or ion-ionreactions) in a reaction or collision cell upstream of the massanalyzer.

Thus, the imaging elemental analyzer may comprise a primary particlesource (e.g. an ion gun) or photon source (e.g. a laser) for generatingthe beam of primary particles and focusing the beam to a localized spoton the surface of the sample to emit secondary particles from the spotand comprises a secondary particle analyzer for analysing the secondaryparticles to determine the presence and optionally quantity of the oneor more elements at the spot, wherein the primary particle source isconfigured to move the spot to a plurality of locations on the surfaceof the sample over time, thereby to obtain an image of the one or moreelements in the sample wherein each location of the spot on the surfaceof the sample corresponds to a pixel of the image. The imaging elementalanalyzer can acquire the image at a rate of at least 100 pixels persecond or at least 1000 pixels per second or in the range of 1000-10000pixels per second.

Preferably, the primary particles are selected from IR or visible lightor UV or X-ray photons, electrons and ions. Also preferably, thesecondary particles are selected photons (especially X-rays), electronsand ions.

Preferably, the energy of the primary particles exceeds 1 keV.

Preferably, the primary particles are ions, and thus more preferably theprimary particle source comprises an ion gun. Preferably, the primaryion beam is continuous. In some embodiments, however, the primary ionbeam is pulsed. Preferably, the primary particles are ions formed at apressure below 1 mbar and the secondary particles are ions for analysisby a mass analyzer. Thus, the imaging elemental analyzer may be animaging secondary ion mass spectrometer (SIMS) configured to be pumpedto a vacuum, wherein the primary particles are ions formed in the sourceat a pressure below 1 mbar and wherein the secondary particles are ionsfor analysis by a mass analyzer of the SIMS.

Scanning of the sample may be implemented by steering plates of the iongun and/or by moving a stage on which the sample or substrate islocated. Higher spatial resolution for sub-cellular and sub-organelleresolution could be achieved, for example, by using a thinner sample(e.g. tissue slice thickness of 3 μm and thinner), and/or strongerspatial focusing of the primary beam and/or lower primary beam currentto reduce space charge defocusing.

Preferably, the beam of primary ions has an intensity of up to 100 nAper 1 μm size of the spot (i.e. 1 μm diameter spot). Preferably, thebeam of primary ions has an intensity of at least: a) 1 pA, or b) 100pA, or c) 1 nA, or d) 10 nA, in a 1 μm size spot.

If photons are used as primary particles, preferably fluence inionisation pulse exceeds a) 5, b) 10, c) 20, d) 50 J/cm² that allowsdense plasma to be formed and high degree of ionisation achieved asknown in the art. A laser device is preferably used as the photonsource. Preferably such laser is a pulsed laser, wherein the pulsespreferably have the aforementioned fluence in each pulse. Any laserwavelength could be used, but preferably not longer than the requiredspatial resolution and preferably with absorbance length (i.e. lengthfor laser intensity to be attenuated by a factor of e (Euler's number))not more than a) 100 nm, b) 200 nm, or c) 500 nm. For example,preferably the fluence in the irradiation pulse exceeds a) 5, or b) 10J/cm² (or the other preferred fluences) and has an absorbance length notmore than 500 nm. Most preferably, solid-state lasers like Nd:YAG areused for ionisation, with or without frequency multiplication (thelatter might be required to obtain sufficiently short absorbancelengths).

Laser radiation could illuminate the sample from the front or from theback. The latter case offers easier focusing of laser beam,orthogonality of the laser light to the sample and hence a roundspot—but high-radiation threshold glass should be used to avoidradiation damage of the slide. In case of front illumination, laserlight will typically come at an angle (to simplify more demandingrequirements of extraction ion optics) while optical observation offocal spot could be done from the back side through the glass slide.

In embodiments where the imaging elemental analyzer comprises a massanalyzer for determining the m/z of emitted ions or their reactionproducts, the mass analyzer may be selected from: a time-of-flight (TOF)mass analyzer, a distance-of-flight mass analyzer, a quadrupole ion trapmass analyzer, an electrostatic trap (EST) mass analyzer (such as anorbital EST, e.g. ORBITRAP mass analyzer), an Ion Cyclotron Resonancemass analyzer (FT-ICR), especially an EST or ICR employing imagingcurrent detection, a magnetic sector mass analyzer, and an array, or anycombination thereof, more preferably the mass analyzer comprises a TOFmass analyzer. The TOF analyzer may be an orthogonal acceleration TOFanalyzer (OA-TOF), especially a high repetition rate OA-TOF as known inthe art, preferably with gridless orthogonal acceleration, optionallywith one or more ion mirrors providing single or multiple reflection ofions in the analyzer (more preferably single reflection). The massanalyzer preferably is capable of analysing a continuous secondary ionbeam over a wide mass range of ions simultaneously, such as TOF orORBITRAP mass analyzer or orbital electrostatic trap. Preferably, thereare a plurality of analyte elements to be analysed (imaged), e.g. from aplurality of elemental tags. More preferably there are at least 5, or atleast 10 analyte elements or elemental tags analysed by the elementalanalyzer. A mass spectrometer, especially a TOF mass analyzer, is easilycapable of such multichannel analysis on short timescales. A combinationof analyzers could be used for multi-modal analysis, e.g. TOF massanalyzer for nominal-mass elemental analysis and an electrostatic trapwith image current detection, such as an ORBITRAP mass analyzer fororganic analysis or high-resolution of interferences.

The secondary ions of the analyte elements (tags) may be emitted into anintermediate vacuum pressure of 10⁻⁵-10⁻² mbar, rather than a highervacuum, thus reducing requirements on sample desiccation and transfertime in comparison to high-vacuum (e.g. SIMS) instruments. Thus, thechamber for housing the sample being irradiated with primary ions has apressure inside the chamber surrounding the sample in the range 10⁻⁵ to10⁻² mbar.

Preferably, the secondary particles for analysis by the mass analyzerare transferred into an RF ion guide after they are emitted from thesurface and are delivered from the ion guide to the mass analyzer. Thus,the imaging elemental analyzer preferably comprises an RF ion guide forreceiving the secondary ions after they are emitted from the surface andtransferring the secondary ions into the mass analyzer. The ion guide inuse is preferably gas filled e.g. to a pressure of at least 10⁻² mbarwhereby all m/z below the mass range of the analyte elements, or theiroxides where the oxide of the elemental ion is detected, are eliminated.The ion guide thereby may advantageously function as a reaction orcollision cell

The ion guide in some embodiments may contain a reactive gas forproducing reaction products with the secondary particles (e.g. viaion-molecule reactions), wherein the secondary particles are ions thatcomprise one or more of the elements. For example, the reactive gas maycomprise a gas such as NO, or O₂ to completely oxidise most of theanalyte elements (e.g. native metals or rare earth elements) to oxidesas known in the art (see e.g. G. Koyanagi, D. Bohme. J. Phys. Chem. A,105 (2001) 8964-8968) and thus reduce interferences (by mass shiftingthose elemental peaks by 16 amu, while other peaks remain unshifted) andincrease the number of channels to be analysed in parallel. If areactive gas is used, all oxides and other molecular interferences couldbe eliminated. Thus, preferably at least some of the produced(secondary) ions undergo ion-molecule reaction in the ion guide. The ionguide can have a cooling effect on the ions as they are transmittedthrough the ion guide. In this way, the energy distribution or emittanceof the ions at the output of the ion guide is changed (preferablyreduced) relative to the ions at the entrance of the ion guide.Accordingly, in some embodiments, a composition and/or an energydistribution of the ions at the output of the ion guide can be changedrelative to the ions at the entrance of the ion guide. The ion guide isthus preferably configured as a reaction cell. It is known in the artthat gases like N₂O, NO₂, O₂, CO₂, NO facilitate oxidation of many metalions, in particular with m/z>100, that in turn allows to reduce a numberof analysis channels per isotope to one. Also, NH₃ and NO could be usedto get rid of polyatomic species like hydrids prior to this oxidation.The reaction cell can therefore be filled with one or more such gasesacting as one or more reactive gases in the reaction cell.

In certain embodiments, as heavier elements get concentrated on theslide surface, they may form a new type of matrix that may start toaffect the analysis resulting in reduced ionisation yield ortopography-related effects. For example, phosphorus and phospholipidsmay concentrate around cell membranes (including nuclear membrane) andfrom DNA—within nucleus. Alkali metals (e.g. Na, K) might appear atdifferent concentrations on the opposite sides of membranes, etc.Therefore, in such embodiments, one or more of these elements (e.g. P,Na, K) should preferably be also monitored and corrections appliedaccording to their abundances. For example, in some embodiments, if theconcentrations of such common elements (e.g. Na, K etc.) do not followtheir expected concentrations in known regions of the sample, then theobserved deviation from their expected concentration can be used as abasis to correct detected abundances of the analyte (typically heavier)elements.

In addition, further treatment could be applied to the sample, e.g.application of additional matrix to enhance ionisation and compensatefor matrix effects, etc. For example, a uniform coating comprising oneor more elements to assist ionisation can be applied to the sample (e.g.oxygen is known to increase a yield of secondary ions of metals). Also,in the case of irradiation and/or ionisation by a laser, an organiclight-absorbing layer can be added to the sample.

Preferably, the mass analyzer is a TOF mass analyzer, especially OA-TOF,and the repetition rate of the TOF mass analyzer is at least or higherthan (a) 5 kHz, or (b) 20 kHz, or (c) 50 kHz, or (d) 100 kHz. Therepetition rate may be 50-100 kHz for example. The repetition rate hererefers to the rate of pulsing ions into the TOF analyzer for separationof the ions by their mass to charge ratio (m/z). A detected ion signal(intensity versus m/z) is obtained from each pulse of ions into the TOFanalyzer. A summation of the detection signals from each pulse of ionsinto the TOF analyzer at a given spot on the sample surface may be usedto generate the corresponding pixel of the image of the elementaldistribution. Preferably, the image of the elemental distribution isacquired at a rate of at least 100 pixels per second or at least 1000pixels per second or in the range of 100-1000 pixels per second.Preferably, the mass analyzer is configured to detect elements or oxidesof the elements.

Preferably, the analysis of each spot using the TOF mass analyzer takesnot more than (a) 2, or (b) 5, or (c) 10 pulses of the TOF analyzer. Theanalysis time of each spot is preferably the time to acquire a massspectrum of a sufficient or desirable signal-to-noise ratio orsensitivity for use in the pixels of the image.

The present invention may find application in any of the following:

-   -   i. tissue imaging, e.g. as applied to anatomical pathology,        especially cancer, including: fixed paraffin-embedded tissues        and in combination with hematoxylin and eosin (H&E) staining;    -   ii. high-content cellular screening;    -   iii. microarray-based targeted assays for clinically relevant        disease biomarkers;    -   iv. high-throughput pharmaceutical, chemical and clinical        analysis;    -   v. bacterial identification and susceptibility testing (with        cultures on a thin layer of media).    -   vi. cytometry, including off-line flow cytometry.

In cases where the sample is a biological sample, especially clinicallyrelevant samples, the method may further comprise using the image of theone or more elements in the sample to determine a physiological state orto diagnose a disease state in an organism from which the biologicalsample is derived.

The sample pre-concentration approach of the invention may providenumerous advantages such as increased pixel acquisition rate andtherefore throughput of elemental imaging; reduced instrumentcontamination; applicability with an expanded range of elemental imagingtechniques; and increased ionisation efficiency due to the presence ofoxidised elemental ion species on the oxidized surface.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an embodiment of an apparatus for imaging oneor more elements in an organic sample in accordance with the invention.

FIG. 2 shows schematically another embodiment of an apparatus forimaging one or more elements in an organic sample in accordance with theinvention.

FIG. 3 shows schematically a pass-through embodiment with transfer ofanalytes of interest onto a counter-slide.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to enable a more detailed understanding of the invention,numerous embodiments will now be described by way of example and withreference to the accompanying drawings.

Referring to FIG. 1, there is shown schematically an apparatus forimaging one or more elements in an organic sample. A thin layer of asample to be analysed is deposited on a slide (2). The sample may bearranged as a microarray on the slide. The slide typically is a flatglass slide with ITO coating. Alternatively, it could be a metal plate.

The sample, for example, is a bio(organic) sample such as a tissuesample or cell line. However, in general the sample is not limited to agiven type. The sample could comprise any one of:

-   -   a biological or a chemical (organic but non-biological) sample    -   a fixed and embedded tissue e.g. Formalin-fixed,        paraffin-embedded (FFPE) tissue, preferably cut by a microtome        to preferably 3-5 μm thickness    -   individual cells deposited on the slide, e.g. from a flow        cytometer or a high-content screening device, for instance in a        grid-like pattern (e.g. deposited every 50 μm in the X and Y        directions, for a typical cell size up to 5-10 μm). Typically,        no more than one biological cell occupies each grid cell or        square. The cells in the grid may all be different (e.g. from        different samples or experiments), or at least some, optionally        all, of the cells in the grid may be from one sample or        population in order to determine a variation among that sample        or population    -   a cell culture on a growth media, e.g. a microbial or bacterial        culture on a thin layer of growth media such as agarose        (preferably, the culture is up to 10 μm, or up to 20 μm thick)    -   a sample, such as non-cellular sample, deposited on the slide,        e.g. by an autosampler of any known type (including by flow        focusing, acoustic droplet ejection, induction, etc.). the        sample may be deposited as individual droplets on a grid-like        pattern (e.g. every 50 μm in X and Y direction), or microarray.

For reasons described further below, in some embodiments, the slide iscoated with a layer of titanium dioxide, for instance in the form of afilm or immobilised particles.

In some embodiments, the sample on the slide could be analysed as it is(i.e. unprocessed, non-tagged) to determine a distribution of nativeheavier inorganic elements (for instance Fe, Zn, Sn, etc., e.g. formetallomics experiments). In other embodiments, the sample could betagged, preferably with one or more elements that are non-native to thesample (herein non-native elements). Rare-earth elements are one knownin the art class of elemental tags. The one or more tags are typicallyselective for one or more different respective targets in the sample.The tags could be applied to the sample before it is deposited on theslide or after but preferably after. The specificity of the tagging canbe achieved with the help of binding members such as antibodies,aptamers, Somamers, metabolic labelling and other known methods. Tagscould include polymer chains, nanoparticles (as shown in U.S. Pat. No.8,679,858), quantum dots, etc. Tagging could also utilise multipleelements in a barcode manner as presented in US 2014/106976 and B.Bodenmiller et al., Nature Biotechnology 30 (2012) 858-867.

The next step in the method of analysing the sample comprises locatingthe slide containing the (tagged) sample in a reaction or oxidationchamber (10), wherein an oxidation reaction is performed to reduce themass of the sample by removal of most of the organic matrix thereby toleave the sample containing the analyte elements of interest, typicallyas oxides. One or both of two approaches to oxidation could be employed.The first approach comprises using light of wavelength <400 nm, such asUV light (12), to irradiate the slide and its sample, preferably atintensities at or above 0.1-10 milliWatt/cm² and wavelength <400 nm(e.g. from a gas discharge source as known in the art). These are theembodiments where it is preferable to include a coating of titaniumdioxide or other photocatalyst on the slide subjected to UV light.Titanium dioxide exhibits very strong photocatalytic properties thatlead to rapid oxidation of matrix atoms. Alternatively, or in addition,a chemical oxidant, preferably ozone or hydrogen peroxide vapour, isadmitted into the oxidation chamber (10) via inlet (14) and is expelledfrom it through an outlet (16). The sample can be heated on the slide toaid the oxidation process as known in the art.

Under the influence of the UV light, and/or the oxidants, a rapidoxidation of matrix atoms occurs: e.g. C→CO₂, N→NO, NO₂, H→H₂O, etc. Thevolatile products are pumped away by a vacuum pump connected to thechamber (not shown), thus carrying away most of the sample mass.Meanwhile, heavier atoms, including the analyte elements of the tags ornative heavier elements, do not form volatile products and thereforeremain on the thinning layer of sample, mainly in an oxidised form. Oncethe process has reached saturation and most of the organic matrix isremoved (preferably >90% or >99% e.g. 90-99% by wt. removed), theheavier atoms are sufficiently concentrated for subsequent analysis. Therate of the oxidation process can be regulated by control of the supplyof oxidants or irradiating light power, and/or temperature of thesample. An oxidation that is too fast undesirably could result in gasbubbles carrying away heavier atoms of interest. Therefore, the rate ofoxidation should be carefully balanced against sample production in thereaction chamber. The volatile products could be used for processcontrol (e.g. to determine the time to stop the oxidation), and/or fordiagnostics (e.g. to measure relative content of elements or theirisotopes to get additional types of information). For example, theelements of the volatile products could be monitored. For instance, ifthe monitored ratio of C to O is 1:1, then this is indicative ofincomplete oxidation, but if the ratio of C to O is 1:2 then this isindicative of complete oxidation to CO₂. In another example, isotoperatios e.g. C¹²/C¹³ could be used as process indicators. For instance,this ratio could be used to distinguish when the process has finishedoxidising a bacteria culture and starts oxidizing the media (which couldhave different ratio C¹²/C¹³ to the bacteria).

An alternative approach is illustrated on FIG. 3 and allows for higherrates of reaction, allowing in principle even boiling and bubbling ofthe sample. In this case one or more oxidizing agents (ozone, hydrogenperoxide, persulfate) are forced (101) from a supply (not shown)underneath the slide (102) that supports a sample (104), for example asample of thin tissue. In this embodiment, there are two spaced apart,but closely separated, sample slides (102,106) each made from porousinorganic material (for example glass, ceramic, ITO etc). The one ormore oxidizing agents are forced through the two closely separatedsample slides. These agents rapidly diffuse through the thin tissuesection producing light gases on the way (i.e. volatile products). Thismixture continues to flow through a 5-10 micrometer gap from the sampleslide (102) to the counter-slide (106) and through the latter. If sampleis sufficiently lyophilised, it could actually brought in direct contactwith the counter-slide (106). The size of pores in the counter-slide ischosen in such a way (preferably, in the range of 1-10 nm) thatnon-volatile heavier elements and their oxides cannot get into the poresand remain on the surface of the counter-slide for subsequent analysis.Thus, imaging analysis, as described herein, can be performed on thecounter-slide. With a sufficiently small gap between the slides, thespatial distribution of the heavier elements in the sample issubstantially preserved. This pass-through approach allows fasteroxidation process without any loss of analytes of interest even whenbubbles are formed. Preferably, the counter-slide is transparent in theUV range to enable ozone formation and titanium dioxide reactions (e.g.where the sample substrate comprises a titanium dioxide surface) aidedby UV radiation from a UV source (108). This can promote oxidationprocesses.

Reaction could be facilitated also by a focused laser rastered acrossthe surface. In one type of embodiments, the power of the laser can behigh enough to create local heating that accelerates the breakdown oforganics and the rate of oxidation.

It should be understood that multiple sample slides could be processedsimultaneously in the oxidation chamber.

For subsequent analysis, the sample is taken from the reaction chamberand transferred to a device capable of rapid imaging of multipleelements in parallel, preferably with an acquisition rate >100, or >1000pixels/second (e.g. 100-1000 pixels/second, or 2000 pixels/second). Therate may be up to 10⁵ pixels/second in some cases. The pixel size forsuch a rate of acquisition may be 10 μm or less, or 5 μm or less, or 2μm or less, or ideally of sub-cellular resolution of 1 μm or less (e.g.0.5-1.0 μm). It should be noted that the reaction chamber could bealternatively integrated with the imaging device, which would bepreferable if, e.g., some means within the latter (e.g. ion or electrongun, or X-ray gun of the imaging analyzer) could be used to acceleratethe oxidation.

A preferred imaging device in the form of a secondary ion massspectrometer (SIMS) or LPI (laser plasma ionisation) mass spectrometeris shown in FIG. 1. It comprises a high brightness laser or ion gun (20)as an irradiation means to produce ions from the sample. Referring toFIG. 2, which shares many of the same features as FIG. 1, there is shownmore detail of an ion gun (20) as a source of primary ions to irradiatethe sample. The ion gun includes ionisation chamber (22), lens system(24), optional collimator system (26), focusing optics (28) to focus theion beam to a small spot on the sample surface, and raster electrodeplates (30) to scan the ion spot across the sample. The primary ion beamionises the elemental tags or native heavier elements of interest andcauses them to be emitted from the surface. Laser ablation (LA) or laserplasma ionisation (LPI) could alternatively be used, preferably at highfluences >1-10 Joules/cm² to facilitate dissociation of molecular bondsand release of ions of elements. Scanning of the continuous primary beam(ion or laser beam) allows to image the sample faster than mechanicalmovement of the sample support enables. Typically, a combination ofrastering by deflection plates over 0.5×0.5 or 1×1 mm area could becombined with mechanical rastering over larger distances, e.g. 50×100 or100×200 mm.

Generally, a continuous high-intensity primary ion beam (36) of up to100 nA in a 1 μm spot is generated. For example, a suitable ion sourceis an oxygen ion beam generated using an RF gas phase ion source asdescribed in N. S. Smith, Appl. Surf. Science, 255 (2008) 1606-1609).This creates secondary ions (38) from the sample surface comprising theelemental tags at an intermediate vacuum pressure of 10⁻⁵-10⁻² mbar inthe chamber that houses the sample, thus reducing, in comparison totypical SIMS instruments, any requirements on sample desiccation andtransfer time. In general, the ions produced from the sample by SIMS orLPI are produced at an intermediate vacuum pressure of 10⁻⁵-10⁻² mbar inthe chamber that houses the sample.

The produced ions (38) (secondary ions in the case of using primary ionsto irradiate sample) are accelerated through a short gas-filledradiofrequency (RF)-driven ion guide or collision cell (40) at elevatedpressure (typically >10⁻² mbar) in which all ions of m/z below the massrange of the tags (or their oxides) are eliminated. The emittance of theion beam is also reduced as the ions pass through the guide. The RF ionguide typically comprises a multipole, such as quadrupole (42) locatedin a gas filled enclosure (44). Such elevated pressure in the RF ionguide or collision cell, unlike in U.S. Pat. No. 7,910,882, allows theuse in the ion guide of an optional reactive gas (especially anoxidising gas, such as NO, O₂ for example) in order to substantiallyoxidise most of the metal ions to oxides as known in the art (see e.g.G. Koyanagi, D. Bohme. J. Phys. Chem. A, 105 (2001) 8964-8968, S.Tanner, V. Baranov, D. Bandura, Spectrochimica Acta B, 57 (2002)1361-1452) and thus reduce interferences by monitoring the oxides of theelements or tags and increase the number of channels to be analysed inparallel. Thus, in certain embodiments, the ion guide is configured as areaction cell. Preferably, the RF ion guide has a DC gradient toaccelerate and control transport of ions.

After passing through the RF ion guide (40), the produced ions (38) ofelemental tags or native elements are mass analysed using a high-speed,orthogonal acceleration (OA) TOF-mass spectrometer (50) as known in theart, but preferably operating at 50-100 kHz repetition rate (i.e. suchthat there are multiple MS scans per spot (pixel) of the sampleionised). Preferably, as shown in FIG. 1, this TOF-MS has a gridlessorthogonal accelerator (52) as described in WO 01/11660, single-stageion mirror (54) and high dynamic range detector (56) comprising anelectron multiplier, e.g. as described in any of: U.S. Pat. No.6,940,066, U.S. Pat. No. 6,864,479, US 2013/264474 or others. Theproduced ions (38) are thereby separated by their m/z and detected asshown. The TOF analyzer enables a wide mass range of ions to be analysedsimultaneously. Preferably, there are a plurality of analyte elements tobe analysed (imaged).

Scanning of the sample surface is implemented by steering (raster)plates of the ion gun or using moving mirror in case of laser and/or bymoving the sample stage (e.g. in x and/or y directions). A higherspatial resolution for sub-cellular and sub-organelle resolution couldbe achieved by using a thinner sample (e.g. a tissue slice thickness of3 μm and thinner), and/or stronger spatial focusing of the primary ionbeam and lower primary beam current to reduce space charge defocusing.

Due to the above mentioned high primary ion current or use of powerfullasers, the secondary ion current from the produced ions could reach upto hundreds of picoamperes (pA) (e.g. up to 100, 200, 500, or 1000picoamperes, or more), with ions of analyte elements (or tags)constituting a significant proportion of the current. This means that anentire pixel could be analysed in just 1 TOF MS pulse, allowing theacquisition rate to approach 10⁵ pixel/sec in some applications (e.g.analysing iron in brain slices). Therefore, an entire slide could beanalysed within a minute, thus greatly reducing the cost per analysis.Images of 500×500 pixels or more may be produced, which are suitable forhistological images. The image may be a 500×500 μm field of the sample(e.g. with a pixel size of 1 μm). Compared to traditional fluorescencedetection of tagged samples, the method of the invention provides areadout equivalent to up to tens to a hundred colours or channels and ata speed comparable with single-colour measurement.

It can be seen from the description above and FIG. 1 that variouspreferred imaging arrangements can be used to image the elements in thereacted or oxidised sample. In one arrangement, laser plasma ionizationcan be used to produce ions from the sample which are caused to enter adownstream reaction cell (wherein the composition and emittance of theproduced ions is altered, preferably towards smaller variation) andthereafter to enter a mass analyzer, preferably a TOF mass analyzer. Thelaser plasma ionization can be scanned across the sample to enable anelemental image of the sample to be obtained. In another arrangement, aSIMS system can be used in which a beam of primary ions is used toproduce secondary ions from the sample, which are caused to enter adownstream reaction cell (wherein the composition and emittance of theproduced ions is altered, preferably towards smaller variation) andthereafter to enter a mass analyzer, preferably a TOF mass analyzer. Thebeam of primary ions can be scanned across the sample to enable anelemental image of the sample to be obtained.

This described method may also provide higher absolute sensitivity ofanalyte element detection approaches due to an increased ion yield dueto the presence of oxidized analyte element atoms on the oxidisedsurface as known in the art. With an ionisation efficiency of 0.1-1% andlow-loss transport under vacuum conditions, the SIMS or LPI method mayprovide an order of magnitude advantage over e.g. LA/ICP-MS due to highlosses (typically, resulting in 1×10⁴-1×10⁵ fold losses) duringtransport in the latter method. On the other hand, the proposed methodof sample treatment is fully compatible with LA/ICP-MS as well, where itcould also reduce sample contamination and carryover.

The results of the analysis could be presented in an analogue orquantitative mode (e.g. determining how much of an element or tag ispresent, as a concentration, preferably taking into account matrixeffects) or in a digital or qualitative mode (e.g. determining if anelement or tag is present or not). The results may be assembled bysoftware into an image, e.g. an image of the elemental distribution (andthus a target distribution where the element was tagged to a target inthe sample).

As an example of a workflow applying the invention, the following stepsare given:

-   -   1) preparing a sample, for example tissue sample,    -   2) oxidizing the sample with light by photocatalysis and/or with        chemical oxidizing agents to remove nearly all of the organic        matrix;    -   3) irradiating the oxidized sample surface by a        continuous-current ion gun or focused laser pulses with a        spatial resolution of 0.5-10 μm with such intensity that the        sample is broken down to constituent elements that are released        from the surface as ions; the irradiation spot is scanned across        the area of the sample surface that it is desired to analyze;    -   4) using a mass spectrometer detecting signals from the ions of        elements in parallel (multichannel detection) for each and every        irradiation spot;    -   5) determining a presence or an absence of spatial distribution        of elements in the sample based on the mass spectrometric        information.

Variations to such a workflow can be made in accordance with thedescription above, e.g. element distributions could be used as proxiesfor corresponding antigens when used as element tags.

As an alternative to the primary ion method of ionisation of the SIMSimaging analyzer of FIG. 2 described above, other ionisation methods invacuum useful for mass-spectrometric analysis could be used: such aslaser plasma ionisation, or laser ablation with laser post-ionisation.It should be noted that laser radiation alternatively could be deliveredfrom the back of the slide, thus utilizing its transparency andsimplifying the optical system.

As alternatives to the mass analysis of ions generated from the samplesurface, other, e.g. non-destructive, methods of elemental imaging couldalso be used for imaging of elemental tags, for example:

-   -   micro X-ray fluorescence (μXRF), which allows analysis at        atmospheric pressure conditions, preferably wherein a        multi-element detector is used to allow parallelisation of        detection    -   X-ray photoelectron spectroscopy (XPS)    -   electron micro probe analyzer (EMPA), especially when integrated        with an electron microscope    -   secondary electron spectrometry (SES)    -   energy dispersive X-ray microanalysis, preferably using silicon        drift detectors

Any of the techniques of elemental imaging could be combined with othermodes of imaging of the sample (e.g. optical imaging). Such opticalimaging could be used as an internal standard for improved quantitationof the sample.

From the above description, the invention can be seen to compriseproviding a substrate or surface with a thin-layer of a (bio-)organicsample that is subjected to an oxidation process that converts organicmatrix, e.g. C, H, N, O, S, into volatile species that enter the gasphase and leave behind mainly heavier inorganic elements in the sample,in particular in oxidised form. The resulting sample is subjected tohigh-speed imaging analysis of the remaining heavier elements by massspectrometry or other techniques in vacuum in order to measure thespatial distribution of the elements in the sample.

In embodiments, the invention enables an elemental imaging massspectrometer, which is capable of delivering subcellular lateralresolution in combination with highly multiplexed sample readout at amuch higher throughput and low cost per analysis. The invention ispreferably based on secondary ion or laser plasma mass spectrometry invacuum with time-of-flight mass analysis (FIG. 1).

The invention finds application in many of today's high-growth marketssuch as:

-   -   tissue imaging, e.g. as applied to anatomical pathology,        especially cancer;    -   microarray based targeted assays for known clinically relevant        disease biomarkers or biomarkers panels and for use in life        sciences research and development;    -   high content cellular screening;    -   high-throughput pharmaceutical and clinical analysis;    -   bacteria identification and antibiotic susceptibility testing.

It will be appreciated that variations to the foregoing embodiments ofthe invention can be made while still falling within the scope of theinvention. Each feature disclosed in this specification, unless statedotherwise, may be replaced by alternative features serving the same,equivalent or similar purpose. Thus, unless stated otherwise, eachfeature disclosed is one example only of a generic series of equivalentor similar features.

The use of any and all examples, or exemplary language (“for instance”,“such as”, “for example” and like language) provided herein, is intendedmerely to better illustrate the invention and does not indicate alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

As used herein, including in the claims, unless the context indicatesotherwise, singular forms of the terms herein are to be construed asincluding the plural form and vice versa. For instance, unless thecontext indicates otherwise, a singular reference herein including inthe claims, such as “a” or “an” means “one or more”.

Throughout the description and claims of this specification, the words“comprise”, “including”, “having” and “contain” and variations of thewords, for example “comprising” and “comprises” etc, mean “including butnot limited to”, and are not intended to (and do not) exclude othercomponents.

Any steps described in this specification may be performed in any orderor simultaneously unless stated or the context requires otherwise.

All of the features disclosed in this specification may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. In particular, thepreferred features of the invention are applicable to all aspects of theinvention and may be used in any combination. Likewise, featuresdescribed in non-essential combinations may be used separately (not incombination).

1. A method of imaging one or more analyte elements in an organic sample, comprising: providing the sample as a layer on a substrate; reacting the sample on the substrate to produce one or more volatile products that leave the sample and enter the gas phase, whilst the one or more analyte elements remain in the sample and are spatially disturbed by the reaction on average not more than the spatial resolution of the imaging analysis, whereby a majority of the sample layer by weight is removed from the substrate by the reaction and the remaining sample layer is enriched in the one or more analyte elements; and subsequently detecting the one or more analyte elements in the enriched sample layer using an imaging elemental analyzer.
 2. The method of claim 1 wherein the sample is a biological sample selected from tissue, cells, bacterial culture and bioorganic solution.
 3. The method of claim 1 wherein the one or more elements are naturally occurring in the sample and/or have been introduced into the sample as elemental tags.
 4. The method of claim 3 wherein the one or more elemental tags comprise one or more rare earth elements, heavy metal elements and/or radioisotopes.
 5. The method of claim 3 wherein the one or more elemental tags are attached to a binding member selected from a polypeptide, polynucleotide, antibody, affibody, and an aptamer, wherein the binding member binds to a target in the sample.
 6. The method of claim 1 wherein the one or more elements are metals or semi-metals.
 7. The method of claim 1 wherein the layer is a thin layer, in particular no more than (i) 20 μm, or (ii) 10 μm, or (ii) 5 μm in thickness.
 8. The method of claim 1 wherein the reaction removes at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%, or at least 99%, or at least 99.9%, or at least 99.99% of the sample layer by weight.
 9. The method of claim 1 wherein the reaction comprises exposing the sample to one or more oxidising agents, optionally wherein the sample is heated during the oxidizing step.
 10. The method of claim 9 wherein the one or more oxidizing agents are selected from (i) electromagnetic radiation and/or (ii) ionic and excited species produced by gas discharge and/or (iii) one or more chemical oxidizing agents.
 11. The method of claim 10 wherein the one or more oxidizing agents are: (i) electromagnetic radiation and the oxidizing step comprises irradiating the sample with light of wavelength less than 400 nm wherein the substrate acts as a photocatalyst to oxidize the sample, or (ii) one or more chemical oxidizing agents and the oxidizing step comprises exposing the sample to one or more chemical oxidising agents selected from: ozone, persulfate and hydrogen peroxide.
 12. The method of claim 1 wherein the substrate is a planar slide, in particular a metal, coated or uncoated glass or coated or uncoated ceramic flat plate, or a substrate having a surface of titanium dioxide.
 13. The method of claim 1 wherein the step of reacting the sample on the substrate comprises passing one or more oxidizing agents from an opposite side of the substrate to the sample through pores in the substrate to reach the sample.
 14. The method of claim 13 wherein there is a second substrate in proximity to but spaced apart from and facing the sample, whereby the one or more oxidizing agents diffuse through the sample producing volatile products from the sample and causing non-volatile analyte elements and/or their oxides to reach and remain on the surface of the second substrate for detection by the imaging analyzer, wherein a gap between the substrates is such that the spatial distribution of the analyte elements transferred to the second substrate is disturbed by the oxidation on average not more than the spatial resolution of an imaging analysis performed by the imaging analyzer.
 15. The method of claim 1 further comprising generating an image of the detected elements in the sample.
 16. The method of claim 15 wherein the imaging elemental analyzer is selected from: a secondary electron spectrometer, an X-ray photoelectron spectrometer, an X-ray fluorescence spectrometer, energy dispersive X-ray microanalyzer, ion mobility analyzer, a radioactivity analyzer and a mass spectrometer.
 17. The method of claim 15 wherein detecting the one or more elements comprises irradiating the enriched sample with a beam of primary particles focused to a localized spot on the surface of the sample to emit secondary particles from the spot and analyzing the secondary particles to determine the presence and optionally quantity of the one or more elements at the spot, wherein the spot is moved to a plurality of locations on the surface of the sample over time, thereby to obtain an image of the one or more elements in the sample wherein each location of the spot on the surface of the sample corresponds to a pixel of the image.
 18. The method of claim 15 wherein the image is acquired at a rate of at least 100 pixels per second or at least 1000 pixels per second or in the range of 1000-10000 pixels per second.
 19. The method of claim 17 wherein the primary particles are selected from X-ray photons, electrons and ions and wherein the secondary particles are selected from photons, electrons and ions.
 20. The method of claim 17 wherein the energy of the primary particles exceeds 1 keV.
 21. The method of claim 17 wherein the primary particles are ions formed at a pressure below 1 mbar and the secondary particles are ions for analysis by a mass analyzer.
 22. The method of claim 17 wherein the beam of primary particles comprises a beam of primary ions and has an intensity of 1 to 100 nA per 1 μm diameter spot.
 23. The method of claim 17 wherein the beam of primary particles comprises a beam of photons from a pulsed laser having a fluence in each pulse that exceeds a) 5, or b) 10, or c) 20, or d) 50 J/cm² and has an absorbance length in the sample of not more than a) 100 nm, or b) 200 nm, or c) 500 nm.
 24. The method of claim 17, wherein secondary particles are ions and the imaging elemental analyzer comprises a mass analyzer, wherein the mass analyzer is selected from: a time-of-flight (TOF) mass analyzer, a distance-of-flight mass analyzer, a quadrupole ion trap mass analyzer, an electrostatic trap mass analyzer, an Ion Cyclotron Resonance mass analyzer, and a magnetic sector mass analyzer or an array or combination thereof.
 25. The method of claim 24 wherein the secondary particles for analysis by the mass analyzer are transferred into an RF ion guide after they are emitted from the surface and are delivered from the ion guide to the mass analyzer.
 26. The method of claim 25 wherein the ion guide contains a reactive gas for producing reaction products with the secondary particles, wherein the secondary particles are ions that comprise one or more of the elements, wherein a composition and/or an energy distribution of the ions at the output of the ion guide is changed relative to the ions at the entrance of the ion guide.
 27. The method of claim 24 wherein the mass analyzer is a TOF mass analyzer and the repetition rate of the TOF mass analyzer is at least or higher than (a) 5 kHz, or (b) 20 kHz, or (c) 50 kHz, or (d) 100 kHz, optionally wherein the analysis of each spot using the TOF mass analyzer takes not more than (a) 2, or (b) 5, or (c) 10 pulses of the TOF analyzer. 28.-43. (canceled) 